Description of the Invention
Electrophoretic coating of resins is predicted upon the migration of charged particles, droplets, or molecules in a conductive medium, to the electrode of the opposite sign, under the influence of an electric current. When this electrode is the workpiece, a coating of the resin is deposited upon it which can be cured to give a strong, adherent film. Since it is an electrical process which is field dependent, this procedure intrinsically seeks out and coats any flaws or pinholes not already insulated and covers points and edges preferentially and efficiently.
While most of the present commercial electrophoretic deposition processes are water based, a need and a preference have arisen for electrophoretic coatings from non-aqueous systems, mainly because an all organic medium allows the use of high performance resins, which is generally not possible in the case of aqueous electrophoretic deposition processes.
Up to now the most commonly and most successfully used high performance resins in a non-aqueous system have been aromatic polyamide-imides and polyimides. These are very expensive materials and are particularly and narrowly suited to low volume, high cost, high performance applications, such as for military purposes. It would be desirable to expand the advantages of this very efficient process to other resins of more general use; however, until now, only relatively low performance resin such as acrylates and alkyd resins, and some low molecular weight epoxies, have been reported to deposit electrophoretically at very high voltages of the order of 10 kilovolts in the presence of pigments (see U.S. Pat. No. 3,450,655). Even at these excessive voltages, the yield, as expressed by deposit thickness and weight, is very low.